Transmission network



Feb. 14, 1933. J KREER, JR 1,897,639

TRANSMISSION NETWORK Filed Feb. 11, 1932 r FIG.

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ery W A TTORIVEV Patented F eb. 14, 1933 UNITED STAES rarar orrics JOHNG. KREER, JR., 0F BLOOMFIELD, NEVT JERSEY, ASSIGNOB "ITO BELL TELEPHONELABORATORIES, IIICOBIE'OBATED, OF NEW YORK, N. Y., A CORPORATION OF NEWYORK TRANSMISSION NETWORK Application filed February 11, 1932. SerialNo. 592,241.

This invention relates to a frequency selective transmission network andmore particularly to a network in which transmission is substantiallyprevented at a predetermined frequency.

In a frequency selective transmission network it is advantageous to havethe resistances of the various meshes contribute directly to the properoperation of the circuit. An appreciable amount of resistance isgenerally unavoidable and if the action of the network is dependent uponthe absence of resistance, the theoretical transmission properties ofthe network will not be realized in practice. If on the other hand theaction of the network is made dependent upon the resistances of thevarious meshes, the theoretical properties of the network can bepractically realized.

In accordance with the present invention a network of three circuitmeshes is provided, each mesh containing reactive elements and resistiveelements. Each circuit mesh is coupled to the other two meshes. The wavesource impedance is included in one of the meshes, and the loadimpedance is included in another mesh. The elements of the network areso proportioned that, at a predetermined frequency, two equalelectromotive forces are induced in the output mesh by any electromotiveforce which may be impressed upon the input mesh. One of these inducedelectromotive forces is transferred directly from the input mesh to theoutput mesh. The second electromotive force is transmitted from theinput mesh to the output mesh through the intermediary of the thirdmesh. 1th the proper adjustment of the circuit elements, as disclosedbelow, the two electromotive forces induced in the output mesh may beadjusted so that at one or more frequencies they are opposite in phaseas well as equal in amplitude. When this balance is obtained, thetransmission from the input to the output of the network issubstantially prevented at a predetermined frequency.

Referring to the attached drawing:

Fig. 1 represents the general schematic form of the network of theinvention;

Figs. 2 and 3 show specific forms of the network of Fig. 1;

Fig. 4 gives typical attenuation characteristics of the networks ofFigs. 8 and 5; and

Fig. 5 shows the network of Fig. 3 in combination with a low-passfilter.

As shown in Fig. l the general schematic form of the network of theinvention is of the bridged-T type, comprising a pair of equalimpedances Z connected in series bet "een two of the network terminals,a bridging branch Z connected between the outer terminals of impedancesZ, and a central branch Z connected between the common terminal ofimpedances Z and the remaining pair of network terminals. The impedancesZ, and Z, represent the terminal loads between which the network may beconnected, and a wave source of electromotive force is represented by E,which is shown connected.

in series with one of the terminal impedances.

The propagation constant P and the characteristic impedance K of thenetwork of Fig. 1 are given by the equations For infinite attenuation itis necessary that the fraction under the radical in Equation equalunity, for which condition ZZ (Z +2Z) (Z+2Z (3) A specific form of theinvention is illustrated-in Fig. 2, in which the two capaciwhere is thequadrantal operator having its usual significance and to is thefrequency, in radians per second, impressed upon the terminals of thenetwork.

Equating the resistances on the two sides of Equation and letting (nequal the frequency of infinite attenuation gives 2R COOZGLQRI.

Equating the reactances on the two sides of Equation gives FromEquations (5) and (6) expressions may be found for R and. C in terms of(n R and L as follows:

p .4600 \OJOLZ 1 the condition for infinite attenuation established byEquation (3) may be applied to the network of Fig. 3 and expressions maybe found for R and L in terms of (0 ,11 and C These equations are R3 (w0 R (D0202 T (L'JO02R4) A typical attenuation characteristic of thenetwork of Fig. 3 is shown by curve A of Fig.

i. The curve starts from Zero at Zero frequency, rises to a maximum atthe frequency of suppression (v and then falls away to a finite value atinfinite frequency. The attenuation at high frequencies is largelydependent upon the value of the bridging resistance R If R is very largethe attenuation will be large at infinite frequency and if R is smallthen the attenuation at the high frequencies will be correspondinglysmall. If P is infinite in value, that is, if the branch containing R isopen-circuited, the bridged- T circuit degenerates into th ordinaryladder-type, constant c, low-pass filter section.

i: For such a filter section the cut-off frequency w is given by theequation It is apparent, therefore, that the shape of the attenuationcharacteristic in the region below the suppression frequency (0 isdependent upon this hypothetical cut'off frequency, which may be sochosen as to give a desired characteristic in the low frequency region.From Equation (2) it may be found that at zero frequency thecharacteristic impedance K of the network of Fig. 3 is given by theequation U The characteristic impedance may, therefore, be given somedesired value at zero frequency by choosing the ratio of L to G Thereare then four variables, L, C R and R and four conditions which may beimposed, two, corresponding to Equations (9) and (10), to provideinfinite attenuation at the suppression frequency m one, presented byEquation (11), to determine the shape of the attenuation characteristic,and one, represented by Equation (12), to fix the impedance at zerofrequency. When these conditions are imposed the values of the elementsmay be computed in terms of the general parameters (0 m and K Theattenuation characteristic of the net work of Fig. 2 is somewhat similarto the one for the network of Fig. 3, discussed above, in that the curvehas its maximum value at the frequency of suppression w Below thisfrequency it falls away to a finite value at zero frequency and abovethe suppression frequency it falls away to zero at infinite frequency.The design in this case also may be determined in terms of the moregeneral parameters representing the characteristic impedance of veryhigh or infinite frequency and the cut-off frequency of the prototypehigh-pass filter obtained by making R infinite and R zero. The high-passfilter out off frequency w and the characteristic impedance at infinitefrequency, denoted by Kw, are given by the equations "a: E K y In thenetwork of Fig. 2, if the values of C and R are fixed the suppressionfrequency and w may, of-course, be adjusted to any Value by ,1

adjusting the values of L and R and the latter two elements may be madevariable, as indicated in the drawing, for this purpose. Or, if desired,L and R may be made the fixed elements and R and the pair of condensersC may be made the variable elements. Likewise, in the network of Fig. 3the elements G and R are shown as variable, so that the frequency (n maybe readily selected.

In Fig. 5 the'suppression network of Fig.

is shown connected in series with a twosection low-pass filter. Theinductances L and the capacitance C of the suppression section may havethe same values as the corresponding elements used in the filtersections. Such a combination may be used, for example, as a filter foruse in conjunction with a rectifier for smoothing out the residualfluctuations of the rectified current. The attenuation peak of thesuppression network may be placed at the frequency of the most prominentcomponent of the current fluctuations thereby suppressing that componentcompletely. Alternatively, the arrangement may be used for sharpeningthe cut-off of the filter. A typical attenuation characteristic of thelow-pass filter portion of the network of Fig. 5 is shown by curve B ofFig. 4, in which to is the cut-off frequency. By placing the suppressionfrequency (t of the bridged-T section close to this cut-off, the attenuation characteristic of the suppression section is made to take theform shown by curve A. The attenuation of the entire network is shown bycurve C, the sharp cut-oft, and steeply-rising attenuation beingattained as the result of the small separation of the frequencies m and(n lVhat is claimed is:

l. A wave transmission network having a pair of input terminals and apair of output terminals, said network comprising an electrical pathbetween each input terminal and a corresponding output terminal, pair ofequal reactances connected in series in one of said paths, a resistanceconnected in parallel with said pair of reactances between the outerterminals thereof and an impedance connected between the junction pointof said pair of reactances and a point in the other of said paths, saidimpedance comprising a second resistance and a third reactance,connected in series relation, the sign of said third reactance beingopposite to the sign of said pair of equal reactances.

2. A bridgedT network comprsing a pair of reactive elements in series, aresistance connected between the outer terminals of said reactiveelements and a central branch connected to the common terminal of saidreactive elements, said central branch comprising a second resistance inseries with a third reac tive element, the reactance of said thirdreactive element being of opposite sign to the reactauce of said pair ofreactive elements, i said two resistances cooperating substani, preventtransmission through said network at a finite frequency.

3. A wave transmission network comprising a pair of input terminals anda pair of output terminals, a resistance connected directly between aninput terminal and an output terminal, a pair of reactances having acommon terminal and having their other term na connected respectively tothe terminals of said resistance, and an impedance path having oneterminal connected to the common terminal .of said pair of reactancesand having connections from its other terminal to the remaining inputterminal and output terminals, said impedance path comprising a secondresistance and a third reactance in series, the sign of said thirdreactance being opposite to the sign of said pair of reactances, saidnetwork having a band of low attenuation and said resistancescooperating to produce complete suppression at one he quency.

4:. A wave transmission networkcomprising four impedance paths arrangedin the form of a bridged-T, two of said paths consisting of one kind ofreactance, the third of said paths comprising reactance of the oppositesign in series with a resistance, and the fourth of said pathsconsisting. of a second resistance, whereby said network effectivelysuppresses waves of a certain frequency.

5. A wave transmission network comprising two capacitances and aninductance arranged in the form of a T, a resistance in series with saidinductance, and a second resistance bridged across said twocapacitances.

6. A wave transmission network comprising two inductances andacapacitance arranged in the form of a T, a resistance-in series withsaid capacitance, and a second resistance bridged across said twoinductances.

7. An electric wave filter having a pair of input terminals and a pairof output terminals, said filter comprising a pair of inductancesconnected in series between an input terminal and a corresponding outputterminal, a resistance connected in parallel with said pair ofinductances between the outer terminals thereof, and an impedance havingone terminal connected to the common terminal of said pair ofinductances and having connections from its other terminal to theremaining input and output terminals. said impedance comprising acapacitance and a secondresistance, said two resistances cooperatin toproduce a peak in the attenuation characteristic of said filter.

8. An electric wave filter having a pair of input terminals and a pairof output ter minals, said filter comprising a pair of capacitancesconnected in series between an input terminal and a corresponding outputterminal, an inductance having one terminal connected to the commonterminal of said pair of capacitances and having connections from itsother terminal to the remaining input and output terminal, and aresistance connected in parallel with said pair of capacitances betweenthe outer terminals thereof, said resistance cooperating with theeffective resistance of said inductance to produce a peak in theattenuation characteristic of said filter. I

9. A wave transmission network having a pair of input terminals and apair of output terminals, said network comprising an electrical pathbetween each input terminal and a corresponding output terminal, aplurality of reactances of like character connected in series in one ofsaid paths, a separa e reactance of opposite sign to said plurality ofreactances connected between each junction point of said plurality ofreactances and a point in the other of said paths, a resistance bridgedfrom a point in one of said plurality of reactances to a point in anadjacent reactance of the same sign, and a second resistance connectedin series with the reactance of opposite sign connected from thejunction point bridged by said first resistance to a point in the otherof said paths, said two resistances cooperating substantially to preventtransmission through said network at one frequency.

10. A plurality of serially connected, ladder-type, low-pass filtersections, each of said sections comprising an inductance in series withthe line and a capacitance in shunt with the line, at least one of saidfilter sec tions being modified by bridging a resistance from a point inone of said inductances to a point in an adjacent inductance and byadding a second resistance in series with the capacitance of the sectionthus bridged by said first resistance whereby complete suppression ofone irequency is effected by the modified filter section.

l1. A plurality of low-pass, ladder-type filter sections connected intandem, each of said sections comprising a pair of input terminals and apair of output terminals, two inductances connected in series between aninput terminal and acorresponding output terminal, a capacitance havingone terminal connected to the common terminal of said two inductancesand having connections from its other terminal to the remaining inputand output terminals, at least one of said filter sections beingmodified by adding a resistance in series with said capacitance and bybridging second resistance across said two inductances between the outerterminals thereof, said two resistances cooperating to produce a peak inthe attenuation characteristic of said modified filter section.

In witness whereof, l hereunto subscribe my name, this 10th day ofFebruary 1932.

JOHN FIR-EEK, JR.

